1. Field of the Invention
The invention relates to a picture pick-up or display device for picking up a scene or for displaying a scene image, the device comprising line deflection circuit for generating a symmetrical line deflection current through a line deflection coil, and a video signal processing circuit having a memory for storing video information and for alternately generating and supplying the video information of a line in one direction and of a subsequent line in the opposite direction, and a clock oscillator for generating a clock signal for reading the video information from the memory, the clock oscillator being incorporated in a control loop which further comprises a counter for counting pulses of the clock signal, and a phase comparison stage and measuring means for receiving a first signal from the line deflection circuit and a second signal from a signal generator, which is also incorporated in the control loop for supplying the second signal at measuring instants, the control loop being adapted to cause the first and the second signal to occur substantially simultaneously at the measuring instants.
2. Description of the Related Art
A picture display device of this type is known from U.S. Pat. No. 4,672,449 (PHN 11.432). In the case of symmetrical line deflection, in which the horizontal lines are successively scanned in the opposite direction, additional measures have to be taken to guarantee that pixels which must lie on a vertical line are indeed displayed exactly underneath each other. This patent describes an embodiment in which, to this end, a first signal corresponding to the zero crossing of the line deflection current is compared with a second signal by means of the phase comparison stage, which signal is a delayed clock signal corresponding to half the video information of a line. Due to the action of the control loop, the delay is controlled in such a way that the first and the second signals substantially conincide so that the video information components corresponding to the centers of the picture lines are displayed exactly underneath each other. However, since the symmetrical line deflection is very sensitive to phase errors, temperature influences and/or ageing phenomena may cause the edges of the picture to be fringed.
Said Patent describes a second embodiment in which delay errors are compensated by means of a second control loop and in which the video information is defined with respect to the line deflection current. The first control loop controls the frequency and the second control loop controls the phase of the second signal. The second signal is generated during the field blanking interval and is not visible.